1. Field of the Invention
The invention relates to signal processing, and, in particular, to the transmission of (e.g., GHz-speed) data through electrical backplanes.
2. Description of the Related Art
Gigahertz-speed data rates are required in core optical products such as high-speed routers and cross-connect switches. Many such large-scale systems require the routing of hundreds to thousands of signals in a small area using little power and for low cost. Typically, this routing occurs on a multi-layer board called a high-speed backplane. Maintaining signal integrity for gigahertz-speed line rates on this structure is very difficult, and has resulted in an important field of study. There are several approaches being pursued by many vendors to maintain backplane signal transmission integrity. These techniques fall into basically two categories: passive and active.
Passive solutions incorporate the use of high-quality microwave substrate materials, innovative via hole techniques, and new connector technology. While these techniques can help address the transmission problem, the use of costly microwave substrates and special high-bandwidth backplane connectors are often required. Moreover, very long trace lengths may still result in less-than-acceptable transmission characteristics.
Active solutions include adaptive equalization, pre-emphasis, PAM-4, and combinations thereof. Although these solutions can provide excellent performance even for long trace lengths, power consumption and cost can be issues. Typically, active solutions that provide equalization or pre-emphasis must correct the entire NRZ data bandwidth. The problem is that, for many low-quality transmission systems, the frequency-response roll-off is severe, and the use of via holes on thick backplanes results in nulls in the frequency range of interest. Equalization or pre-emphasis through nulls requires the use of higher-order networks, and the resulting correction will be very sensitive to temperature and parameter variations.
One solution to the problem of poor high-frequency response is to compress the bandwidth using multi-level coding. PAM-4 is currently being used with equalization by some vendors to address this problem. Although this technique has been shown to provide very good performance even over long traces, these circuits are typically complex, leading to difficulty providing dense integration and significantly increased power consumption relative to standard NRZ signaling.